The invention concerns an electrically driven device for angular adjustment of a camshaft relative to a crankshaft of an internal combustion engine, said device comprising an eccentric gearing comprising at least one internally geared wheel and one spur gear that meshes with the internally geared wheel and is adapted to be driven by an electrically rotatable eccentric shaft.
DE 41 10 195 C2 describes a generic electrically driven device for the angular adjustment of a camshaft relative to a crankshaft of an internal combustion engine. This device comprises an eccentric gearing that comprises at least one internally geared wheel and one spur gear meshing with the internally geared wheel, which spur gear can be driven by an eccentric shaft that is rotatable by an electromotor.
With regard to the number, configuration and design space requirement of its components, and particularly of its gears, this device is complex and expensive to manufacture. Besides this, noise and vibration problems are encountered due to the large number of meshing teeth and the lacking balance of masses.
The above applies substantially also to the electrically driven adjusting device described in DE 41 33 408 A1. Although a mass balance is provided for the outer eccentrics, these possess a high mass moment of inertia that necessitates a corresponding amount of adjusting work.
It is an object of the invention to improve a generic electrically driven device for the angular adjustment of a camshaft relative to a crankshaft of an internal combustion engine so that the device has a simple structure and a small design space requirement while, at the same time, the noise and vibration level is lowered. This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves the above objects by the fact that the internally geared wheel is configured as a first and a second internally geared wheel, and the spur gear is configured as a first and a second spur gear that have the same number of teeth and are rotatable in opposite directions, the internally geared wheels are connected to the crankshaft and the spur gears are connected to the camshaft and can be driven through a double eccentric shaft comprising identical eccentrics arranged offset at 180xc2x0 to each other. The configuration as divided internally geared wheels and spur gears permits a rotation of these in opposite directions for compensating flank clearance, while the identical eccentrics offset at 180xc2x0 effect the balancing of masses. This results in a low-noise and low-vibration operation of the eccentric gearing. The direct connection of the internally geared wheels to the driving component and the spur gears to the driven component leads to the formation of a space and cost saving single-stage adjusting gearing.
Due to the fact that the internally geared wheels and the spur gears have the same number of teeth, it is possible to manufacture each of these in larger numbers economically in a single pass (e.g. by broaching).
Due to the osculation of the outer contour of the spur gears with the inner contour of the internally geared wheels, the degree of overlap is not limited only to one or two teeth as in the case of common-type toothed gears but lies between 0.15 to 0.2 of the total number of teeth. For this reason, despite the small module, a high torque can be transmitted. Besides this, in most cases, a hardening of the teeth can be dispensed with. The small module also permits a very compact structure of the eccentric gearing.
It has proved to be of advantage that a rotationally fast connection of the spur gears to a driven shaft that is fixed to the camshaft is realized through a separable coupling. This enables a simple assembly and disassembly of the adjusting device.
Advantageously again, the separable coupling is configured preferably as a pin coupling comprising driving pins that are pressed into axially parallel shaft bores of the driven shaft and engage positively into axially parallel spur gear bores of the spur gears.
In place of the pin coupling, it is also possible to use a segment coupling or an Oldham coupling in which projecting transmission elements of one side of the coupling engage into corresponding recesses of the other side of the coupling. None of these couplings have circumferential backlash but through their axial and radial play, they can compensate for tolerances.
Due to the fact that the diameter of the spur gear bores is equal at least to the diameter of the driving pins augmented by twice the eccentricity of the eccentrics, the pin coupling can be plugged together in a simple manner. A further important pre-requisite for this is the correspondence of the pitch circle diameter and the pitch of the spur gear bores and the shaft bores.
According to another important provision of the invention, elimination of tooth flank clearance and of the play between the driving pins and the spur gear bores is achieved by the fact that the first and second internally geared wheels can be braced together with a cover by flange screws that can be screwed into the second internally geared wheel and that the flange screws have a larger clearance in the first internally geared wheel than in the cover. An elimination of play can be achieved in that, with loosened flange screws, the internally geared wheels are held and rotated slightly against each other by a tool that engages into the pin bore of the cover and the notch of the first internally geared wheel. The circumferential backlash required for this purpose is present in the through-holes for the flange screws in the first internally geared wheel. In this way, the elimination of circumferential backlash can be effected in the installed state of the adjusting device from its side situated away from the camshaft.
Alternatively, an abutment of the driving pins in the spur gear bores and of the tooth flanks against each other can be effected and the flank clearance thus eliminated or reduced to a desired size, for example, by an electromotive rotation of the double eccentric shaft during which the camshaft is held fast and the internally geared wheels are loosened but also held fast. Following this, the flange screw connection must be tightened so as to fix this state.
A minimization of the bearing friction in the device is achieved in that the spur gears, the double eccentric shaft and the driven shaft are preferably mounted in rolling bearings. However, the rolling bearings can also be replaced at least partly with oil-drenched bronze or plastic bearings. The increase of friction brought about by this favors the achievement of self-locking. Besides this, sliding bearings reduce the overall size and structural complexity. However, self-locking is influenced, above all, by an appropriate choice of the transmission ratio.